The present invention is directed to a printing device. The printing device is particularly usable to print materials that are curved around a printing cylinder.
The present invention is directed to a printing device that is usable for imprinting material to be imprinted, which material is curved around a printing cylinder. The printing device has a plurality of print heads and a print head holder, which holder is used for seating the print heads on a holding plane defined by the print head holder. The print heads are arranged along the print head holder in a row which extends at an acute angle in respect to the printing direction.
xe2x80x9cPrinting directionxe2x80x9d in this context means the running direction of the material to be imprinted in relation to the print heads. If the material to be imprinted travels around or wraps around the printing cylinder, the printing direction extends perpendicularly in relation to the axis of rotation of the printing cylinder.
As a rule, ink jet printers operate with two print heads, namely with one print head for color printing and with one print head for black printing. A movement along two axes takes place between the material to be imprinted and the print heads. The material to be imprinted is moved in the printing direction, and the print heads are moved transversely to this, so that the image points can be exactly aligned with each other by utilization of a chronological control of the shot times. As a rule, printing of flat material takes place, so that positional deviations of the individual image points in respect to their neighboring image points are small, and accordingly there is also only a slight distortion of the printed image.
However, the disadvantages of these systems are their limited printing speed and the requirement for exact control of the shot time of the printing nozzles in relation to the axes of movement. The latter is complicated, because the movement of the print head in relation to the material to be imprinted is multi-axial.
For achieving a high printing speed, together with a simple control, a plurality of print heads can be arranged next to each other for covering the entire printing width. The movement between the print heads and the material to be imprinted is single-axial, only a movement of the material to be imprinted in the printing direction takes place. To achieve a compact arrangement of the print heads, and accordingly a high resolution, the individual print heads can be placed obliquely in relation to the printing direction and can be arranged offset from each other in the printing direction. The print points of the print heads complement each other and cover the entire width of the material to be imprinted.
To make possible a simple mounting and arrangement of the print heads, it is desirable to always fasten at least two of the print heads on a common level print head holder. However, in this connection, it is difficult to prevent distortions of the printed image and to achieve a high print quality, in particular when printing on curved material to be imprinted.
GB 2 349 607 A shows a printing device with a plurality of print heads which are arranged offset in respect to each other. These print heads are arranged inclined in the radial direction with respect to a printing cylinder.
USP 4,864,328 shows an ink jet print head with several rows of nozzles. These rows of nozzles are inclined in respect to the printing direction.
The object of the present invention is directed to providing a printing device of compact arrangement and with a high printing quality.
In accordance with the present invention, this object is attained by providing a printing device that is usable for imprinting material which are curved around a printing cylinder. The printing device has a plurality of print heads and a print head holder for supporting the print heads on a holder plane. Nozzle openings in the print heads are arranged along a row which is inclined in respect to the printing direction. The holder plane is also inclined with respect to the printing direction. The print heads are arranged on the holder so that they are substantially perpendicular with respect to the material to be imprinted.
The advantages which can be obtained by the present invention lie, in particular, in that the printing device prints at a high speed, achieves a high print quality and prevents distortions of the printed image. The print heads are inclined in relation to the holder plane in such a way that the nozzles in the print heads spray substantially vertically or perpendicularly onto the material to be imprinted. Because of the inclined arrangement of the print heads, it is possible to use a level print head holder. At the same time, the print heads can print substantially vertically on corresponding set points. Distortions of the printed image are prevented, and it is possible to achieve an outstanding print quality even at the highest printing speed.
The print heads can be inclined on the print head holder, relative to the holder plane, as a function of their distance. In particular, the inclination of the print heads is set as a function of the distance between the respective print heads and the holder center. The holder center means the section of the print head holder which is located closest to the printing cylinder and which is perpendicularly intersected by a straight diameter line through the printing cylinder. If the print heads are arranged in pairs symmetrically with respect to the holder center, the distance of the print heads from the holder center corresponds to one-half the distance of the print heads from each other, so that the inclination of the print heads can be set as a function of the distance of the print heads from each other.
In a further development of the present invention, the print heads are inclined in relation to the holder as a function of the acute angle by which the print heads, or a row of nozzles of the print head, are placed obliquely, or at which the row of print heads extends in relation to the printing direction. The obliquely placed print heads of a row are fastened on a common print head holder, so that the longitudinal direction of the print head holder corresponds to the orientation of the row, and accordingly the inclination of the print heads in relation to the print head holder is set as a function of the acute angle of the print head holder in relation to the printing direction.
To achieve a simple adjustment of the print heads in relation to the print head holder, the print heads are inclined at two different angles on two planes extending perpendicularly in respect to each other. They can be inclined, in particular, on the holder plane and on a plane perpendicularly thereto. The directions of the angles to be set are predetermined, regardless of the angle of inclination actually to be set, because of which the preconditions for mounting can be simply and precisely preset and executed.
In accordance with an advantageous embodiment of the present invention, the print heads can be inclined at a first angle on a plane which is defined by a straight connecting line through the print heads on the respective print head holder and the straight diameter line through the printing cylinder, which intersects this straight connecting line. Thus the first angle is located on a plane which contains the longitudinal direction of the print head holder and which cuts an ellipse out of the printing cylinder. In this case, the first angle of each print head is selected in such a way that the spraying direction of the respective print head extends perpendicularly on a tangent line at a set point against the ellipse which is cut out of the printing cylinder by the plane.
In a further development of the present invention, the print heads are each inclined relative to the holder plane at a second angle on a plane which extends perpendicularly in respect to the straight connecting line through the print heads which are mounted on a print head holder, and which also extends through the respective print head. Thus the second angle lies on a plane perpendicularly in respect to the holder plane.
To be able to imprint a respectively wide material to be imprinted over its entire width, and without displacing the print heads transversely to the printing direction, a plurality of print head holders are arranged parallel with each other, each of which print head holders supports a plurality of obliquely placed print heads. The inclination of the print heads in relation to each print head holder is set in the manner previously described.
A preferred embodiment of the present invention is represented in the drawings and will be described in greater detail in what follows.
Shown are in:
FIG. 1, a top plan view of a printing device in accordance with the present invention and with a printing cylinder, a print head holder and with two print heads seated thereon in a schematic representation,
FIG. 2, a sectional view taken along the line Bxe2x80x94B in FIG. 1,
FIG. 3, a side elevation view of the printing device, taken in a direction of view parallel with the longitudinal extension of the print head holder, and in
FIG. 4, a front view of the printing device taken in a direction of view parallel with the axis of rotation of the printing cylinder.